Controlling bondary layer fluid flow

ABSTRACT

A method of controlling fluid flow ( 14 ) in a boundary layer at a fluid-surface interface comprising, providing a plurality of blades ( 11 ) which project from the fluid contracting surface into a boundary layer such that in use the blades ( 11 ) are orientated to control fluid flow ( 14 ) in the boundary layer.

This invention relates to the control of fluid flow in a boundary layerat a fluid-surface interface, especially controlling turbulent flow.

The control of fluid flow in the boundary layer can have the effect ofreducing, or increasing, friction or surface drag at a fluid-surfaceinterface. In particular, the invention is concerned with the control ofturbulent fluid flow in the boundary layer.

This invention has particular application at the fluid-surface interfaceof vehicles, in particular fluid craft, by which is meant any craftwhich moves through a fluid such as cars, road vehicles, trains,aircraft, watercraft, ships, underwater vessels, hovercraft, balloons;and in relation to pipes or conduits carrying air, oil or other fluidswhere the control of the fluid flow and attendant friction or surfacedrag is a concern. However, it can be applied to any situations wherethere is a fluid-surface interface, such as wind turbine blades, gasturbine blades or a swimsuit.

A boundary layer of fluid surrounds any solid body or surface which hasrelative movement in relation to a fluid with which it is in contact -such as, an aircraft in the air, or a pipe carrying gas or liquid. Morespecifically, the boundary layer is the layer of fluid between a surfaceand a main stream fluid flow over the surface. The relative velocity ofthe surface and fluid at the fluid-surface interface is zero. There is atransition of velocities through the boundary layer adjacent the surfaceas one moves away from the surface towards a main stream fluid flow,until the main stream fluid flow velocity is reached.

The nature of the fluid flow in the boundary layer determines the degreeof surface friction or drag at the solid surface. Turbulent flowproduces significant surface friction or drag, which can be more thantwice as much as that when fluid flow at the boundary layer is laminar.

Whenever a body moves through a viscous medium, or indeed a viscousmedium moves through or over a body, drag forces will reduce themechanical efficiency of the system. Efficient operation of suchsystems, be they aircraft, hydrodynamic vehicles or pipelines,necessitates that these drag forces be as low as possible.

The total drag acting on a surface can be separated into the componentspressure drag, induced drag and, for high mach numbers, wave drag. Forstreamlined bodies at subsonic speed, the major component of drag is dueto skin friction.

In order to reduce drag or surface friction, say in an aircraft, it isdesirable to reduce turbulent flow in the boundary layer and toencourage more laminar flow. The reduction of surface friction on theouter surface of an aircraft allows improved fuel efficiency, forexample up to 50% of fuel burnt on a commercial airliner is used toovercome skin friction. The increased fuel efficiency may result in anincreased passenger/cargo capacity, faster flights, and even the abilityto use shorter runways, as well as a reduction in noise levels andstructural fatigue. A balance between these advantages is usuallystruck.

A reduction in drag or surface friction can also be used to reduce heattransfer at the fluid-surface interface, protecting structures fromextremes of temperature.

In other circumstances, it may be desirable to increase drag or surfacefriction. For example, some aircraft use devices known as vortexinducers or generators to increase lift during take-off.

Much research has been undertaken to address the manipulation of fluidflow in the boundary layer, in particular to reduce surface friction ordrag in aircraft. This research can be broadly split into two areas,namely passive and active control. Passive techniques attempt to imposea broad-scale global control on the turbulent boundary layer withoutenergy input, to obtain global skin friction reductions. Active controlrelies on sensing and then interacting with the turbulent fluid flow ata local level, the aim being to reduce local skin friction whilstpossibly having broader effects on the global regenerative mechanism.Some prior art of which we are aware is listed below.

U.S. Pat. No. 4,706,910 (Walsh et al) describes a passive system of flowcontrol which results in reduced skin friction on aerodynamic andhydrodynamic surfaces. Surface friction or drag is reduced by acombination of two devices, namely: (i) a series of ‘riblets’ or small,flow aligned ‘v’ micro-grooves with dimensions of 0.05 to 0.5 mm,intended to reduce disturbances in fluid flow near wall surfaces, inparticular to reduce wall vortices and turbulent burst dimensions; and(ii) large eddy break up (LEBU) devices configured as small aerofoils orflat ribbons, parallel to or spanwise across the airflow, extending 50to 80% of the thickness of the boundary layer, that is some 7.5 to 15mm, intended to cause a disruption of the large scale vortices.

U.S. Pat. No. 5,848,769 (Fronek et al) and WO89/11343 (Choi) alsodescribe surface friction or drag reducing devices configured asriblets.

Hefner, Weinstein & Bushnell (1979) Prog. Astronaut Aeronaut 72, 110-127describe tests using 22.86 cm spanwise arrays of one, two or threehorizontal elements, supported by four 7.62 cm vertical elements. Noparametric analysis of the vertical elements was undertaken, which wereconsidered to be provided only for support purpose.

Savill & Mumford (1988) J. Fluid Mech. 191, 389-418, describe studiesusing LEBU devices configured as horizontal elements extending parallelto the surface. They were tested at various heights and chords, stackedand in tandem.

Yajnik & Acharya (1977) in Structure and Mechanisms of Turbulence,Lecture Notes in Physics, vol: 76, 249-260, describe LEBU devicesconfigured as small honeycomb fences of approximately boundary layerheight, which result in a 50% c_(f) (skin friction) reduction. However,the net drag is observed to increase by several hundred percent.

LEBU devices have been applied to aircraft to reduce drag or surfacefriction during flight. Such devices are generally configured as smallairfoils or horizontal devices, suspended from the aircraft outer frame,and extending parallel to the surface of the aircraft and orientatedacross the direction of fluid flow. Generally, LEBU devices are locatednear the edge of the boundary layer to disrupt the large eddies.

The requirement that LEBU devices are suspended from a surface resultsin problems of device rigidity and security. If configured as thinsheets LEBU devices tend to flutter if not sufficiently supported.However, the more supports introduced or any increase in devicethickness will be to the detriment of device drag. Indeed at highReynolds numbers the preferred design of LEBU devices switches to anaerofoil section (low drag, high stiffness structure), with associatedcomplications due to sensitivity of profile shape, angle of attack andchord Re number.

The Reynolds (Re) number is defined as Re= Ux/v, where U is the flowspeed, x is the length of the body and v is the kinematic viscosity offluid. The Reynolds number of the boundary layer over the aircraft is‘high’, compared to wind tunnel or laboratory experiments, because U(aircraft speed) and x (body length) are greater on an aircraft.

The chord Reynolds number is as described above except x is the chordlength of the blades rather than the length of the body upon which theblades are located.

The UK Patent Office has undertaken a novelty search on the presentinvention and identified U.S. Pat. No. 5,988,568, DE 3534268, DE3609541, U.S. Pat. Nos. 4,425,942, US 4,836,473, US 5,734,090 and GB1034370 which in general relate to devices and methods for inducingvortex formation to allegedly reduce drag at a fluid surface interface.

Skin friction reductions have also been realised by injecting polymerchains into fluid flows to interrupt the near wall structures, or byinjecting micro-bubbles into a liquid flow. Alternatively, skin frictionmay be reduced by oscillating the surface in a spanwise direction oreven oscillating the flow in the spanwise direction, for example, usingLorenz force control of sea water.

According to an aspect of the present invention we provide a method ofcontrolling fluid flow, in a boundary layer at a fluid-surface interfacecomprising: providing a plurality of blades which project from a fluidcontacting surface into a boundary layer, such that in use the bladesare orientated to control fluid flow in the boundary layer.

Preferably, the blades are self supporting.

In a preferred configuration the blades are orientated to straighten thefluid flow, and accordingly are orientated generally aligned with thedirection of fluid flow. In this configuration the blades comprise flowmanipulator blades which ‘comb’ and ‘straighten’ turbulent fluid flow inthe boundary layer. As a result, fluid flow downstream of the blades isless turbulent, than it was upstream of the blades, and the friction orsurface drag created by turbulent fluid at the fluid-surface interfaceis reduced, in comparison with the same surface without blades.

Alternatively, the blades may be orientated to induce turbulence orgenerate vortices in the fluid flow. More specifically, this may beachieved by orientating the blades, in particular those on the wingand/or stabilisers, at an angle across the direction of fluid flow toinduce turbulence or vortices in the fluid flow. This may increasesurface friction or drag at the surface.

In a preferred method the blades are applied to the fluid contactingsurface of a vehicle, such as an aircraft, or the fluid contactingsurface of a fluid carrying conduit, such as a pipe.

Preferably a reduction in surface drag or friction will reduceaerodynamic noise and reduce structural fatigue as well as realising aweight saving. Typically, heat transfer will result as a consequence ofreduced surface friction or drag thus affording structures/surfaces towhich the blades are applied some protection from extremes oftemperature. Preferably an at least 2%, 5%, 10% or 15% improvement inreduction of surface drag; reduction of noise levels; reduction of fuelconsumption; or increased speed; is observed compared to vehicle,including an a aircraft, without flow manipulator blades projecting fromthe fluid contacting surface.

At least one hundred blades may be used. Alternatively at least onethousand blades may be used. Alternatively, at least ten thousand bladesmay be used.

According to a further aspect of the invention, we provide a boundarylayer flow control apparatus comprising a surface, over which fluid canflow in a boundary layer; and a plurality of blades projecting from thesurface, the blades being configured such that in use they are capableof controlling the flow of fluid within the boundary layer.

In a preferred embodiment the blades are aligned with the expecteddirection of the fluid flow, and are in use capable of straightening thefluid flow in the boundary layer, thereby reducing surface friction ordrag in comparison with the same surface without such flow controlapparatus.

Alternatively, the blades are orientated at an angle across the expecteddirection of the fluid flow, and are capable in use of inducingturbulence or vortices in the fluid flow in the boundary layer, therebyincreasing surface friction or drag in comparison with the same surfacewithout flow control apparatus.

In a preferred configuration the blades may be mounted substantiallyvertically on the surface, configured as flat plate elements which aregenerally rectangular. Preferably the blades have a constant crosssection across the length and/or the width of the blade. Furthermore,the blades may be mounted generally parallel and be of uniform heightand/or width, and/or chord, and/or spacing, and/or orientation and/ordimensions and/or rigid in use. Alternatively the blade dimensions mayvary across a surface.

Preferably, the blades project into the boundary layer by 100 to 200wall units, for example between about 25% and about 50% of the boundarylayer depth. The wall units are non-dimensional units based on the localinner flow conditions, h⁺=hu*/v, where h⁺ is the non-dimensional bladeheight, h is the actual height, u* is the friction velocity, and v isthe kinematic viscosity. The blade may be 1 mm high, have a 1 mm chordand be spaced by 1 mm. Preferably the ratio of blade height to width tochord is selected from the following list:

-   1:1:1, 1:2:1, 1:3:1, 1:4:1, 1:5:1, 1:6:1-   2:1:1, 2:2:1, 2:3:1, 2:4:1, 2:5:1, 2:6:1-   3:1:1, 3:2:1, 3:3:1, 3:4:1. 3:5:1, 3:6:1-   4:1:1, 4:2:1, 4:3:1, 4:4:1, 4:5:1, 4:6:1-   5:1:1, 5:2:1, 5:3:1, 5:4:1, 5:5:1, 5:6:1-   6:1:1, 6:2:1, 6:3:1, 6:4:1, 6:5:1, 6:6:1-   1:1:2, 1:2:2, 1:3:2, 1:4:2, 1:5:2, 1:6:2-   2:1:2, 2:2:2, 2:3:2, 2:4:2, 2:5:2, 2:6:2-   3:1:2, 3:2:2, 3:3:2, 3:4:2, 3:5:2, 3:6:2-   4:1:2, 4:2:2, 4:3:2, 4:4:2, 4:5:2, 4:6:2-   5:1:2, 5:2:2, 5:3:2, 5:4:2, 5:5:2, 5:6:2-   6:1:2, 6:2:2, 6:3:2, 6:4:2, 6:5:2, 6:6:2-   1:1:3, 1:2:3, 1:3:3, 1:4:3, 1:5:3, 1:6:3-   2:1:3, 2:2:3, 2:3:3, 2:4:3, 2:5:3, 2:6:3-   3:1:3, 3:2:3, 3:3:3, 3:4:3, 3:5:3, 3:6:3-   4:1:3, 4:2:3, 4:3:3, 4:4:3, 4:5:3, 4:6:3-   5:1:3, 5:2:3, 5:3:3, 5:4:3, 5:5:3, 5:6:3-   6:1:3, 6:2:3, 6:3:3, 6:4:3, 6:5:3, 6:6:3-   1:1:4, 1:2:4, 1:3:4, 1:4:4, 1:5:4, 1:6:4-   2:1:4, 2:2:4, 2:3:4, 2:4:4, 2:5:4, 2:6:4-   3:1:4, 3:2:4, 3:3:4, 3:4:4, 3:5:4, 3:6:4-   4:1:4, 4:2:4, 4:3:4, 4:4:4, 4:5:4, 4:6:4-   5:1:4, 5:2:4, 5:3:4, 5:4:4, 5:5:4, 5:6:4-   6:1:4, 6:2:4, 6:3:4, 6:4:4, 6:5:4, 6:6:4-   1:1:5, 1:2:5, 1:3:5, 1:4:5, 1:5:5, 1:6:5-   2:1:5, 2:2:5, 2:3:5, 2:4:5, 2:5:5, 2:6:5-   3:1:5, 3:2:5, 3:3:5, 3:4:5, 3:5:5: 3:6:5-   4:1:5, 4:2:5, 4:3:5, 4:4:5, 4:5:5, 4:6:5-   5:1:5, 5:2:5, 5:3:5, 5:4:5, 5:5:5, 5:6:5-   6:1:5, 6:2:5, 6:3:5, 6:4:5, 6:5:5, 6:6:5-   1:1:6, 1:2:6, 1:3:6, 1:4:6, 1:5:6, 1:6:6-   2:1:6, 2:2:6, 2:3:6, 2:4:6, 2:5:6, 2:6:6-   3:1:6, 3:2:6, 3:3:6, 3:4:6, 3:5:6, 3:6:6-   4:1:6, 4:2:6, 4:3:6, 4:4:6, 4:5:6, 4:6:6-   5:1:6, 5:2:6, 5:3:6, 5:4:6, 5:5:6, 5:6:6-   6:1:6, 6:2:6, 6:3:6, 6:4:6, 6:5:6, 6:6:6

Blade height, chord or spacing may be between 2 and 10 mm. Blade widthmay be about 0.1 mm, alternatively the blades may be 0.2 to 10 mm thick.Blade height may be 0.5 mm. Alternatively, blade height may be between0.6 and 10 mm high.

Blades may have a chord of 0.5 mm. Alternatively, blades may have achord of 0.6 to 10 mm. The blades may be spaced by 0.3 mm.Alternatively, blades may be spaced by 0.4 to 10 mm.

The blade height and/or chord and/or spacing may vary over the surfaceupon which they are mounted.

In a further embodiment the blade orientation can be adjusted relativeto the direction of fluid flow, indeed it may be that the bladeorientation can be adjusted to maintain a fixed orientation relative tothe direction of fluid flow.

Preferably blades may be actively controlled, such that their locationcan be alternated between a positive (aligned with the fluid flow) and anegative (orientated across the fluid flow) angle of attack. Counterrotation of the blades on an aircraft can be used to energise theboundary layer and prevent separation occurring at certain points in theflight envelope, such as stall separation at high aircraft incidences.When separation control is not required the blades can be realigned tothe flow to give skin friction reduction. To allow blade rotation someadjustment of streamwise spacing may be required.

Actively adjustable blades also provide directional control allowing forlocal or selective steering of fluid flow. For example, on an aircraft,by reducing skin friction on one wing and increasing skin friction onthe other local yawing moments can be produced. Similarly, themanipulation of air flow around the stabilisers can produce pitchingmoments.

Whilst in the preferred arrangements discussed above the blades areconfigured as thin rectangular elements, mounted extending directly awayfrom a surface, alternative configurations comprising various bladeshapes and angles of projection are envisaged.

In a further embodiment of the invention, an array of blades isenvisaged, with at least one row of parallel blades. However, as thestraightening effect of the blades on the fluid flow boundary layer isonly transient, turbulence may begin to re-appear in the flow after thefluid has flowed a significant distance past a row of blades. Thus,repeated rows of blades may be employed, spaced to prevent significantturbulence re-emerging in the fluid flow. In a preferred blade array,this spacing is some 50 to 100 times blade height. Alternatively, therows of blades may be spaced by 80 mm to 200 mm in the streamwisedirection.

Preferably, the array of blades comprises at least two rows of blades.The first row comprising a plurality of parallel blades aligned with thedirection of fluid flow, and the second row also comprising a pluralityof parallel blades aligned with the direction of fluid flow. Preferablythere are no blades in the gap between the two rows of blades.Preferably, blades in the first row share a substantially commonlongitudinal axis with blades in the second row.

This is in contrast to the riblets described in U.S. Pat. No. 4,706,910,which must be applied over the entire surface where a reduction in dragor surface friction is sought. Furthermore, the configuration of theriblets as small ‘v’ grooves (of 0.05-0.5 mm) results in problems ofdebris or dirt becoming lodged therein, resulting in high maintenancedemands.

According to a further aspect, the invention provides a surface uponwhich is mounted a boundary layer flow control apparatus according tothe invention.

Preferably the surface is on a vehicle, such as a plane, or on a pipe.

In a yet further embodiment of the invention, flow manipulator bladeelements are provided mounted on a strip or patch, which can beincorporated on a surface during article or surface manufacture, or canbe applied to an existing surface, for example, blades may beretrofitted to a surface on a vehicle or in a pipe. In particular, theblades may be applied to the surface of an aircraft. Alternatively, theblades may be applied to the fluid-surface interface of a pipe or anyfluid-carrying conduit.

On an aircraft, with a body, wing and tail sections, the boundary layercontrol apparatus may be mounted upon the body, wing, and/or tailsections.

In a pipe, the boundary layer flow control apparatus may be mounted onthe internal surface. Preferably the pipe has a central axis about whichthe flow manipulator blades are radially located, extending inwardstowards the central axis. The blades may be located as one discreteband, or multiple discrete bands, on the internal surface of the pipe.

According to a still further aspect, the invention provides an aircraftwith a boundary layer flow control apparatus according to the inventionmounted upon the surface wherein the blades are moveable between a firstconfiguration, in which the blades are orientated to straighten fluidflow in the boundary layer, and a second configuration, in which theblades are orientated to induce turbulence in the boundary layer.

According to another aspect, the invention provides a method of reducingthe surface drag of an aircraft having an outer surface skin comprisingaffixing a large number, preferably at least five hundred, of flowmanipulator control blades to the surface skin, the blades being alignedwith the expected direction of fluid flow past the aircraft skin.

Alternatively, at least one thousand blades may be at least a fixed tobe surface skin, or at least ten thousand blades may be affixed to thesurface skin.

According to another aspect, the invention provides a method of reducingthe surface drag in a pipe or conduit having an inner surface comprisingaffixing flow manipulator control blades to the inner surface, theblades being aligned with the expected direction of fluid flow past thesurface.

The blades of the subject invention are self-supporting and thereforeare to a large degree free of the constraints of the LEBU devices thatrequire suspension.

When the blades are configured to reduce surface friction or drag, andare flow aligned, any device drag will be minimal, the device thicknessbeing sufficiently low to give low form drag.

A reduction in surface friction or drag is observed when using flowaligned vertical blade elements due to a number of effects which includedisruption of lifted longitudinal vortices associated with the near wallstructure. Also near field disruption of longitudinal vortices isobserved. Further from the wall/surface the blade elements interact withthe head and neck of hairpin or horseshoe vortices, cancelling andunwinding them to reduce surface friction or drag. The blade elementsalso have a plate effect and a wake effect which inhibits spanwiseturbulent motions in the boundary layer, hence reducing wall normal andlongitudinal vorticity components.

It will be appreciated that the optional features discussed in relationto any aspect of the invention may apply to all aspects of theinvention.

Embodiments of the invention will now be described in more detail by wayof example with reference to the accompanying drawings, of which:

FIG. 1A shows schematically the location of a boundary layer;

FIG. 1B shows a schematic perspective view of a surface upon part ofwhich is mounted a row of flow manipulator blades;

FIG. 2 shows a schematic view from above of an array of flow manipulatorblades, similar to those of FIG. 1;

FIG. 3 is a schematic perspective view of flow manipulator bladesapplied to an aircraft;

FIGS. 4A and 4B show flow manipulator blades applied to the internalsurface of a pipe;

FIG. 5 is a schematic perspective view of flow manipulator blades asused in fluid flow experiments;

FIG. 6 depicts alternative blade spacing, width and height to thosedepicted in FIG. 5;

FIGS. 7 and 8 show graphically the effect of varying flow manipulatorblade spacing on surface friction levels for various blade heights;

FIGS. 9 and 10 shows graphically the effect of flow manipulator bladeheight on surface friction levels for various flow manipulator bladespacings;

FIG. 11 shows graphically the effect of flow manipulator blade chord onthe surface friction levels;

FIG. 12 shows a perspective schematic view of flow manipulator bladesmounted in a row upon a strip;

FIGS. 13A and 13B show perspective schematic views of flow manipulatorblades mounted in rows upon a patch;

FIGS. 14A to 14D show schematic views from above of alternative arrayconfigurations of flow manipulator blades;

FIG. 15 shows a schematic view of a flow manipulator blade positionedperpendicular to the direction of fluid flow;

FIGS. 16A to 16C show schematic views of a movable blade;

FIG. 17 shows a schematic representation of an aircraft fitted with‘intelligent’ flow manipulator blades;

FIGS. 18A to 18H show alternative flow manipulator blade geometries;

FIGS. 19A to 19D show variant flow manipulator blade mounting angles;and

FIG. 20 depicts a series of pins for use in manipulating boundary layerfluid flow.

FIG. 1A shows schematically the flow 4,6, of fluid flow over a surface 3to illustrate the location of the boundary layer. Essentially, there isa main stream flow of fluid 4 over a surface 3. Upon contact with thesurface the flow is disrupted at the fluid-surface interface. This layerof disrupted air flow 6, between the surface 3 and the mainstream flow4, is known as the boundary layer 8. The depth of the boundary layervaries depending upon relative velocity and direction of the airmovement, and the viscosity of the fluid

FIG. 1B depicts a perspective view of an array 17 of flow manipulatorblades 11 mounted upon a surface 13. Disruption 15 of the general fluidflow 14 at the fluid 14—surface 13 interface is illustrated.

The surface 13 is depicted divided into six zones, three locateduppermost on the surface 21, 22 and 23, and three lowermost 24, 25 and26. Zones 21, 22 and 23 illustrate the effect of flow manipulator blades11 on fluid flow 14 in the boundary layer. By way of contrast, zones 24,25 and 26 illustrate fluid flow over a clean flat planar surface withoutflow manipulator blades.

Considering firstly the lowermost zones 24, 25 and 26 of the surface13—which represent the clean surface, as fluid flow 14 passes over thesurface in zone 24, turbulence 15 begins to appear in the boundarylayer. As fluid flow progresses through zones 25 and 26 the turbulence18 increases. This increased turbulence 18 results in increased surfacefriction or drag.

In contrast, the uppermost zones 21, 22 and 23 illustrate the effect offlow manipulator blades 11 on fluid flow in the boundary layer. As shownin the lower zone 24, when fluid flow passes over the surface 13 in zone21 the fluid flow is disrupted and turbulence 15 begins to occur in theboundary layer. As the turbulent fluid flow 15 enters zone 22 and passesthrough the vertically mounted parallel, thin, rectangular bladeelements 11, mounted in an array 17 upon the surface 13, the flow isstraightened and becomes more laminar 16 in nature.

In this embodiment the blades 11 are configured as an array 17 ofparallel blades 11 positioned in a row, each blade 11 being aligned withthe direction of fluid flow 14—that is, at a zero angle of attack to thefluid flow. The blades 11 are equally spaced, positioned at right anglesto the surface 13 and all have constant height, chord and width.

However, the straightening effect on the flow is only transient, andturbulence will begin to develop again some distance downstream of theblades 11, as illustrated in zone 23 where turbulence 19 is beginning toreappear in the generally laminar flow 16.

FIG. 2 further illustrates the transience of the straightening effect ofthe flow manipulator blades 11′ on the fluid flow 14′. A first array orrow 17′ of flow manipulator blades 11′ is depicted (which is similar tothe array 17 of FIG. 1), together with a second array or row 29 ofblades 11″, parallel to the first array 17′. This second array 29 islocated downstream of the first array 17′ and is positioned wherepreviously straightened fluid flow 16′ begins to become disrupted andturbulent 19′ again. The second array 29 serves to re-straighten thefluid flow, maintaining a more laminar flow 16″ over a greater length ofsurface 13′. For example, when the blades extend by 100 to 200 wallunits into the boundary layer, it is anticipated that a second row ofblades will be positioned about 50-100 times the blade heightdownstream. The longitudinal axis of the first blade 11′ is insubstantially the same plane as the longitudinal axis of second blade11″.

By straightening the flow of fluid in the boundary layer, turbulence isreduced, and friction or surface drag at the fluid-surface interface isdecreased.

The reduction of boundary layer fluid flow turbulence, and hence drag orsurface friction, has of long been a concern in aircraft design. It isenvisaged that the flow manipulator blade elements subject of thisinvention will be suitable for mounting upon the outer surface of anaircraft to reduce friction.

FIG. 3 illustrates a schematic aircraft 32 highlighting possible regions34 where turbulence and friction may be a problem, and where flowmanipulator blades 31, aligned with the expected direction of fluidflow, would serve to straighten fluid flow and reduce friction. Foreconomic reasons it is unlikely that flow manipulator blades would bemounted over an entire aircraft surface, it is unlikely that blades willbe mounted on areas which experience predominantly laminar flow, such asaround the nose, the forward fuselage and the front sections of thewings, tail and stabilisers. More likely fluid flow manipulator blades31 will be applied only to those regions 34 where drag or surfacefriction is a problem. Indeed, it is likely that the blades will bespaced as discussed in FIG. 2 to overcome the transience of thestraightening effect thereby producing a striped or ‘lemur tail’ effectin regions 34. Exploded view 36 illustrates an area of the aircraftsurface 37 and shows two spaced rows 38,38′ of blades 31. In addition,blades may also be located upstream of an air intake in order to improvethe efficiency of the intake.

It anticipated that some 10,000s of flow manipulator blades will beapplied to a aircraft, with rows of blades typically spaced by 80 to 200mm and located predominantly on the rear of the wings, nose, tail, andstabiliser and along the length of the fuselage with the exception ofthe nose and most forward regions. The large number of blades to be usedmeans the loss or damage to any one blade would likely have nosignificant impact on the overall effect of the blade arrays.

Typically, on an aircraft, blades will be configured to be flow alignedwhen the aircraft is cruising. For most aircraft the flow vectorcorresponding to direction of fluid flow over various parts of theaircraft is known, as indeed are the appropriate laminar/turbulenttransition points.

FIGS. 4A and 4B illustrate an alternative practical use for the flowmanipulator blades 41 described, and that is mounted on the interiorsurface 42 of a pipe 43. Indeed they could be located on thefluid-surface interface of any fluid carrying conduit, such as an openchannel.

In more detail, FIG. 4A illustrates a pipe 43 upon which are mounted, onthe inner surface 42, flow manipulator blade elements 41 (not visible inFIG. 4A). The blades are orientated to be aligned with the fluid flow inthe pipe, and serve to straighten fluid flow in the boundary layer atthe fluid-surface interface. The blades are located as series of bands44 at spaced intervals along the length of the pipe 43, downstream bandsbeing employed to re-straighten fluid flow before significant turbulencere-appears. It is envisaged that a pipe carrying water of diameter 1.2 mwith a flow rate of 1.5 m³/s will have blades of dimension 3.6 mm spacedat 1.8 mm intervals.

FIG. 4B depicts a cross section along IV-IV of FIG. 4A. Flow manipulatorblade elements 41 project from the inner surface 42 of the pipe 43 andinto the fluid flow—more specifically the blades are located radiallyabout the central axis of the pipe, extending inwards towards thecentral axis. The blades 41 are orientated to be aligned with directionof fluid flow.

These examples of practical uses of the flow manipulator blades are byno means exhaustive, the blades could be employed to reduce or increasefriction wherever there is a fluid-surface interface.

FIGS. 5 through 11 depict the results of wind tunnel experimentsundertaken to study the efficiency in modifying surface friction levelsof various dimensions and spacings of a row of flat plate parallelrectangular blade elements, flow aligned (zero angle of attack) andmounted vertically to a surface.

The air speed used in the wind tunnel for these experiments was 2.5ms⁻¹, which is significantly lower than the airspeed passing over anaircraft during flight.

The reduced air speed in the wind tunnel experiments requires largerblades to be used than would be necessary at higher fluid velocities. Itis anticipated that when applied to an aircraft the blades will have achord, height and spacing of only several millimetres. Typically, on alarge passenger aircraft blades will be arranged from the start of theboundary layer in spanwise arrays, with a spanwise spacing of 70 to 150wall units and a height of 100 to 200 wall units.

Thus, for an aircraft cruising at a velocity of 269 ms⁻¹, with an airviscosity of 3.5303e-5 m²s⁻¹ and a fuselage length from transition(where turbulent air flow begins to appear) to trailing edge ofapproximately 50 m, surface friction or drag could be reduced by usingblades ranging in height from approximately 0.7 mm at transition to 1 mmby the end of the fuselage, with a characteristic spacing ranging from0.3 to 0.4 mm. The blades being repeated in the streamwise directionapproximately every 80 to 200 mm depending on precise location andoptimisation. This in comparison to the much smaller ‘riblet’ deviceswhich are typically approximately 50 microns in height and spacing, andthe much larger LEBU devices which range from a location 20 mm from thewall in the forward position and 0.4 m from the wall at the tail end ofthe fuselage.

The data from the wind tunnel experiment can be scaled to apply at anygiven air speed using the scaling law/design rule h⁺=hu*/v.

All blades used in the wind tunnel experiments are made from −0.3 mm(0.012 inch) plastic or steel shim. Thickness is not considered to playa major part in skin friction reductions, but may play a large role whenconsidering overall device drag—the thinner the device the less thedevice drag.

FIG. 5 is useful in explaining the dimension nomenclature used insubsequent studies to describe the flow manipulator blade geometry andspacing. The studies consider the parameters of:

-   -   blade height h—height of the blade in the surface(wall)-normal y        direction; and    -   blade chord c—length of the blade in streamwise x direction;    -   blade packing—spacing between blades in the spanwise z        direction.

Thus, a blade described as 30x10z20 would have:

-   -   blade height h of 30=30 mm;    -   blade chord c of x10=10 mm;    -   blade packing of x20=20 mm.

In the wind tunnel experiments the blades 51 are mounted in slottedbrass pegs 52 flush with the test surface (not shown in this figure).

In the wind tunnel experiments discussed below, the flow manipulatorblades 51 are aligned with the direction of fluid flow 54.

FIG. 6 illustrates examples of alternative flow manipulator bladespacing, height and chord dimensions, as used in subsequent experiments.In the wind tunnel experiments blade elements are mounted on 10 mm pegs55, and can therefore be spaced at a minimum of 10 mm intervals. 10 mm(z10 ), 20 mm (z20), 30 mm (z30) and 60 mm (z60) spacing is illustrated.Various chord and height dimension combinations are depicted, forexample, 60×15 represents a blade with a height of 60 mm and a chord of15 mm.

FIGS. 7 through 11 illustrate the results of parametric studiesundertaken in the wind tunnel, according to the conditions describedpreviously, to study the effect of blade geometry on skin frictionlevels. The studies examine effects up to 740 mm downstream from theblade trailing edge. Downstream locations are denoted on the graph asx(mm).

Spanwise Packing of Blades

FIGS. 7 and 8 consider the effect of spanwise packing, that is, relativespacing, of the blades on skin friction levels observed at thefluid-surface interface.

FIG. 7 illustrates graphically the effect of spanwise flow manipulatorblade packing on averaged c_(f) reductions for blade height h=30 mm andchord=15 mm. Blade packing is varied to include 10 mm, 20 mm, 30 mm and60 mm spacing.

Skin friction is recorded using the c_(f) measurement technique whichgives comparative skin friction results to <±1.5 % error, and isdescribed in Hutchins and Choi, AAIA (American Institute of Aeronauticsand Astronautics) Paper-2001-2914. C_(f) is proportional to the velocitygradient near the body surface, and is determined by taking an accuratemeasurement of velocity near the wall in order to determine C_(f)values. Essentially, c_(f) can be regarded as a measure of skinfriction, and the terms are used interchangeably.

The percentage c_(f) reduction is determined at intervals downstreamfrom the trailing end of the device up to 740 mm. The width of the studyarea is 60 mm.

The results show that the skin friction (percentage c_(f)) reductionincreases for increased spanwise packing (that is the blades are closertogether). This perhaps is not altogether surprising since as spanwisepacking increases more material is being put into the path of theflow—more frontal area, more surface area and more wake is being putinto the boundary layer.

For example, consider z60 (60 mm blade spacing), which in this case,over a 60 mm by 740 mm area, is a single 30×15 (30 mm high and 15 mmchord) blade element, and a reduction of approximately 2.6% in the c_(f)is observed.

In contrast, for the z10 (10 mm blade spacing) spanwise packing, withsix 30×15 blade elements, in the same 60 mm by 740 mm area, a 24%reduction in the c_(f) is observed—somewhat more than six times the z60reduction. Thus, spanwise packing cannot be considered as a simpleadditive process, but that closer packed arrays are more effective atreducing surface friction.

FIG. 8 illustrates graphically for a smaller range of variables theeffect on c_(f) (skin friction) values of varying the spanwise packingbetween 10 mm, 20 mm and 30 mm, for flow manipulator blades with a fixedchord c of 15 mm and a height h of 20 mm (rather than the 30 mm in FIG.7). Again, a similar trend in skin friction reductions is noted, thecloser the blades the greater the percentage c_(f) reduction.

Blade Height

FIG. 9 illustrates the effect of blade height of surface frictionlevels, in general an increase in blade height results in a reduction insurface friction.

In more detail, FIG. 9 illustrates graphically the effect of varying theblade height, between 5 and 60 mm on the percentage c_(f) reduction,data was taken at various intervals from the trailing edge of the deviceto 740 mm downstream. The chord c is fixed at 15 mm and the spanwisepacking is fixed at 10 mm spacing. The percentage c_(f) reductionobserved increases with blade height, over at least the first 740 mm, toa limit of (blade height) h=30 mm, after which additional c_(f)reductions are minimal for further blade height increase.

FIG. 10 illustrates graphically a similar effect on skin friction levelsto that of FIG. 9 when blade height is varied for spanwise packing ofz20 (20 mm blade spacing). Consistent with FIGS. 7 and 8 the overallmagnitude of the peak c_(f) reductions is considerably lower than FIG. 9due to the increased spanwise spacing.

Again, an increase in percentage c_(f) reduction is seen with increasingblade height, up to a limit of (blade height) h=30 mm, at least in theregion up to 740 mm downstream of the blade array. In fact, bladeheights of 30, 40 & 60 mm all look quite similar, especially if a ±1%accuracy on c_(f) measurements is included. Further downstream (beyond740 mm) persistence of the effect is not analysed.

Blade Chord

FIG. 11 illustrates graphically the effect of flow manipulator bladechord c on spanwise averaged c_(f) reductions for spanwise packaging z10(10 mm blade spacing) and height h=30 mm. The blade chord is variedbetween 5 and 50 mm. c_(f) levels are recorded at intervals up to 740 mmdownstream of the blade array trailing edge.

As the blade chord is increased from 5 to 50 mm there is a correspondingincrease in skin friction (c_(f)) reduction.

Application of Flow Manipulator Blades

Flow manipulator blades may be incorporated onto a surface duringmanufacture or retrofitted to an article, that is, fitted to a surfacepost-production. This would allow the blades to be fitted to an aircraftalready in service, or to be added to pipes after manufacture but beforethey are laid.

Blades may be applied individually, or as a group. FIG. 12 illustratesan array of parallel rectangular blades 71, mounted horizontally on astrip or tape 72 ready for attachment as a row 73 upon a surface, suchas the wing of an aircraft.

Alternatively, blades may be mounted as an array 75 on a patch 76, asillustrated in FIG. 13A, ready for attachment to a surface. Spacing ofthe parallel rows 77, 77′ is optimised for use to prevent there-appearance of turbulence in fluid flow that has already beenstraightened by the forward row of blades 77. FIG. 13B depicts analternative to that of FIG. 13B in which the blade height increases ineach row 85, 86, 87 across the surface 88, that is blade 81 is higherthan blade 82 which is higher than blade 83. Spacing of the rows isoptimised to reduce the re-appearance of turbulent fluid flow. Blades81, 82 and 83 share a common longitudinal axis.

FIG. 14A to 14D illustrate, in plan, various schematic blade arrays.FIG. 14A illustrates an array 92 of flow manipulator blades 91configured as two parallel rows 93 of individual flow manipulator blades91. The individual blades 91 are orientated in line with the directionof fluid flow 94.

By way of contrast, FIG. 14B depicts an alternative array 95 in whichindividual flow manipulator blades 91′ are arranged in two parallelchevrons 96. Individual blades 91′ are orientated in line with thedirection of fluid flow 94′.

A yet further array 97 is depicted in FIG. 14C. In this case individualblade elements are arranged in two parallel diagonal rows 98. Individualblades 91″ are orientated in line with the direction of fluid flow 94″.

A still further array 100 is depicted in FIG. 14D configured as twoparallel rows 99, 99′ of individual flow manipulator blades 91′″. Theindividual blades 91′″ are orientated in line with the direction offluid flow 94′″. In contrast to FIG. 14A the first row 99 and second row99′ of blades are offset somewhat.

In each case, two rows or two chevrons are illustrated, the first rowserving to straighten fluid flow upon passage over the blades, and thesecond row or chevron is intended to re-straightens flow in whichturbulence has begun to reappear. Whilst the illustrations depict onlytwo rows, in practice any number of rows could be employed.

If blades are to be fitted to an aircraft, or indeed any rivetedsurface, if may be convenient to manufacture the rivets to include aflow manipulator blade, possible integrated with the rivets (notillustrated).

As well as reducing drag or surface friction by the straightening offluid flow by using flow aligned blades to produce more laminar flow inthe boundary layer. It may be desirable in some circumstances to disruptthe fluid flow in the boundary layer and thereby increase turbulence,and thus increase drag or surface friction.

By adjusting the angle of attack of the blade 101 to cross the fluidflow 103, as depicted in FIG. 15, the blade can serve to induceturbulence or vortices 105 in the fluid flow, thereby increasing drag orsurface friction. This may be desirable say to increase the lift of anaircraft during take-off and landing.

In some circumstances it may be desirable to alter the use to which theblades are put, for example, skin friction could be reduced on one wingof an aircraft and increased on the other to produce yawing moments, orincreased on the stabilisers to produce pitching moments.

Furthermore, the use of flow manipulator blade elements that can bemoved to a desired angle of attack is envisaged. FIGS. 16A, 16B and 28Cillustrate a movable (in this case rotatable) blade element. In FIG. 16Athe blade 101′ is configured to have an angle of attack across,perpendicular to, the fluid flow 103′ and thereby induce turbulence 105′in the fluid flow. By way of contrast, in FIG. 16B the blade 101″ hasbeen rotated such that is now aligned, parallel, with the fluid flow103″, the blade 101″ serves to straighten the fluid flow and the fluidflow downstream of the blade 101″ is more laminar 106 in nature. FIG.16C shows a further variant in which the blade 101′″ is configured tohave an alternative angle of attack across the fluid flow 103′″, againturbulence 105″ is induced.

Blade rotation could be manually controlled or computer controlled inresponse to a sensor system.

For any given surface the fluid flow in the boundary layer around thatsurface will vary depending on a number of factors, including the flowspeed, the surface angle, the temperature, proximity to the surfaceedge, the nature of the fluid etc′.

For optimal efficiency in reducing surface friction or drag, an array offlow manipulator blades can be located on a surface to align with thepredicted flow of fluid over the surface. For example, if blades are tobe applied to a vehicle, the alignment will be optimised for aparticular speed while travelling through a particular medium—say, foran aircraft travelling at 9144 metres (30,000 feet) at a speed of 650kilometres per hour the typical properties of air encountered, includingair viscosity, temperature, path of flow over the surface, at thatheight are known, thus the blades can be aligned accordingly to reducesurface friction by reducing turbulence in the boundary layer.

The resulting configuration is unlikely to be a row of parallel bladesthe length of the surface, but this could be a satisfactoryapproximation.

The angle of attack of the aligned blades with respect to the fluid flowwill depend upon whether a reduction or an increase in drag or surfacefriction is sought.

In a more sophisticated variant, an array of blades may be configured toalign themselves to the local fluid flow. A series of sensors may belocated, for example, to the fore of the blades which are capable ofdetermining the direction of fluid flow. In response to thisinformation, which may be forwarded to a central processing unit foranalysis, the blades can be continually tuned to the local fluid flow.

FIG. 17 depicts the use of ‘intelligent’ blade arrays 111 on an aircraft112. Sensors 114 located on the body of the aircraft collect informationregarding the direction of local airflow, which is relayed to a centralprocessing unit 115 for analysis. In response to the local informationreceived, the alignment of the blades can be automatically adjusted.When the aircraft is cruising it is intended that all blades will beflow aligned, to straighten air flow in the boundary layer and reducesurface friction. However, on take-off or landing it may be desirable toincrease friction or surface drag on some areas of the aircraft, say toincrease lift or to slow down, in these circumstances the appropriateblades can be rotated to be at an angle to the local flow to induceturbulence in the boundary layer.

As well as rectangular blade elements, other geometries could beemployed as flow manipulator blades, examples are illustrated in FIGS.18A through 18H, which depict various blade geometries, such astriangular 121, 125, 126 square 122, parallelogram elongated in thehorizontal 123 or vertical 124, rectangular with a numbered leading edge127, and rectangular with a sharpened trailing edge 128. This list isnot exhaustive. Alternatively, the blades may be configured as aerofoilsections (not illustrated).

The blade elements discussed previously are all mounted vertically 131,at 90° to the surface, as illustrated in FIG. 19A. The blades couldhowever be mounted at a more inclined angle 132 of less the 901, anexample of such is depicted in FIG. 19B. Alternatively, blades could bebent or curved 133 as in FIG. 19C or sinusoidal 134 as in FIG. 19D.

FIG. 20 depicts a series of pins 138 which could be used, eithersingularly or in series, as an alternative to the blades discussedabove. A row of pins can constitute a ‘blade’.

It is further envisaged that the device of the subject invention couldbe used in combination with other skin friction modifying techniques,including vortex generators, LEBU devices, riblets, compliant coatings,polymers/surfactants and/or micro-bubbles.

1-22. (canceled)
 23. A method of controlling fluid flow in a boundarylayer at a fluid-surface interface comprising: providing a plurality ofblades which project from a fluid contacting surface into a boundarylayer such that in use said blades are orientated to control fluid flowin said boundary layer.
 24. A method according to claim 23, wherein saidblades are orientated to straighten said fluid flow.
 25. A methodaccording to claim 23 wherein said blades are orientated generallyaligned with the direction of fluid flow to straighten said fluid flow.26. A method according to claim 23 wherein said blades are orientated toreduce the drag or surface friction at said fluid contacting surface.27. A method according to claim 23 wherein said blades are orientated toinduce turbulence or vortexes in said fluid flow.
 28. A method accordingto claim 23 wherein said blades are orientated at an angle across thedirection of said fluid flow to induce turbulence or vortexes in saidfluid flow.
 29. A method according to claim 23 in which said fluidcontacting surface is that of a vehicle or fluid carrying conduit.
 30. Aboundary layer flow control apparatus comprising: a surface, over whichfluid can flow in a boundary layer, and a plurality of blades projectingfrom the surface, said blades being configured such that in use they arecapable of controlling said flow of fluid within said boundary layer.31. A boundary layer flow control apparatus according to claim 30 inwhich, said blades are aligned with the expected direction of said fluidflow, and in use are capable of straightening said fluid flow in saidboundary layer, thereby reducing surface friction or drag in comparisonwith the same surface without flow control apparatus.
 32. A boundarylayer flow control apparatus according to claim 30 in which said bladesare orientated at an angle across the expected direction of said fluidflow, and are capable of inducing turbulence or vortexes in said fluidflow in said boundary layer in use, thereby increasing surface frictionor drag in comparison with the same surface without flow controlapparatus.
 33. A method of controlling fluid flow according to claim 23,in which said blades are mounted extending substantially directly awayfrom said surface.
 34. A boundary layer flow control apparatus accordingto claim 30, in which said blades are mounted extending substantiallydirectly away from said surface.
 35. A method of controlling fluid flowaccording to claim 23, in which said blades are selected from the groupconsisting of: (a) configured as flat plate elements; (b) generallyrectangular; (c) generally parallel; (d) generally of uniform height;(e) generally of uniform width; (f) generally of uniform chord; (g)generally of uniform spacing; (h) generally of uniform orientation; (i)generally uniform dimensions; and 0) dimensions vary across a surface.36. A boundary layer flow control apparatus according to claim 30, inwhich said blades are selected from the group consisting of: (a)configured as flat plate elements; (b) generally rectangular; (c)generally parallel; (d) generally of uniform height; (e) generally ofuniform width; (f) generally of uniform chord; (g) generally of uniformspacing; (h) generally of uniform orientation; (i) generally uniformdimensions; and (0) dimensions vary across a surface.
 37. A method ofcontrolling fluid flow according to claim 23, in which said bladesproject into said boundary layer by 100 to 200 wall units.
 38. Aboundary layer flow control apparatus according to claim 30, in whichsaid blades project into said boundary layer by 100 to 200 wall units.39. A method of controlling fluid flow according to claim 23, in whichsaid blade orientation can be adjusted relative to the direction offluid flow.
 40. A boundary layer flow control apparatus according toclaim 30, in which said blade orientation can be adjusted relative tothe direction of fluid flow.
 41. A method of controlling fluid flowaccording to claim 23, in which said blades are arranged as an array ofmultiple repeated rows.
 42. A boundary layer flow control apparatusaccording to claim 30, in which said blades are arranged as an array ofmultiple repeated rows.
 43. A method of controlling fluid flow accordingto claim 23, in which said blades have a height, width and chord ratioof X:Y:Z wherein X is between 1 and 6, Y is between 1 and 6 and Z isbetween 1 and
 6. 44. A boundary layer flow control apparatus accordingto claim 30, in which said blades have a height, width and chord ratioof X:Y:Z wherein X is between 1 and 6, Y is between 1 and 6 and Z isbetween 1 and
 6. 45. A method of controlling fluid flow according toclaim 29 in which at least a 2% improvement in one or more selected fromthe group consisting of: a) reduction of surface drag; b) reduction ofnoise levels; c) reduction of fuel consumption; and d) increased speed;is observed compared to a vehicle, including an aircraft, without saidflow manipulator blades projecting from said fluid contacting surface.46. A method of controlling fluid flow according to claim 29 in which atleast a 5% improvement in one or more selected from the group consistingof: a) reduction of surface drag; b) reduction of noise levels; c)reduction of fuel consumption; and d) increased speed; is observedcompared to a vehicle, including an aircraft, without said flowmanipulator blades projecting from said fluid contacting surface.
 47. Amethod of controlling fluid flow according to claim 29 in which at leasta 10% improvement in one or more selected from the group consisting of:e) reduction of surface drag; f) reduction of noise levels; g) reductionof fuel consumption; and h) increased speed; is observed compared to avehicle, including an aircraft, without said flow manipulator bladesprojecting from said fluid contacting surface.
 48. A method ofcontrolling fluid flow according to claim 29 in which at least a 15%improvement in one or more selected from the group consisting of: i)reduction of surface drag; j) reduction of noise levels; k) reduction offuel consumption; and l) increased speed; is observed compared to avehicle, including an aircraft, without said flow manipulator bladesprojecting from said fluid contacting surface.
 49. A surface upon whichis mounted a boundary layer flow control apparatus according to claim30.
 50. An aircraft, with body, wing and tail sections, with boundarylayer flow control apparatus as claimed in claim 30 mounted upon thebody, wing and/or tail section.
 51. A pipe with an internal surface uponwhich is mounted boundary layer flow control apparatus as claimed inclaim
 30. 52. A method of reducing the surface drag of an aircrafthaving an outer surface skin comprising affixing a large number,preferably at least five hundred, of flow manipulator control blades tothe surface skin, said blades being aligned with the expected directionof fluid flow past said aircraft skin.
 53. A method of reducing thesurface drag in a pipe or conduit having an inner surface comprisingaffixing flow manipulator control blades to said inner surface, saidblades being aligned with the expected direction of fluid flow past thesurface.